Breast cancer is the most prevalent malignancy among women. More than 200,000 cases of breast cancer were diagnosed in the United States in 2010 (Cancer Facts & Figures, 2011, American Cancer Society). The lifetime risk of getting breast cancer for any particular patient in North America is about 1 in 7.
On a more promising note, breast cancer awareness and screening have allowed most breast cancer cases to be diagnosed at an early-stage. Most clinicians agree that early detection of breast cancer saves thousands of lives each year. Screenings are important as breast cancer does not cause obvious symptoms until the cancer has grown and possibly spread beyond the breast, significantly lowering the treatment success rate. At an early stage, however, breast cancer is smaller and still confined within the breast, thereby enabling an easier and more effective treatment. The current standard of treatment for most early-stage breast cancers is breast conservation therapy (BCT), consisting of lumpectomy followed by 6-8 weeks of radiation therapy.
The predominant method used for breast cancer screening is mammography. A mammogram is an x-ray projection of the breast, usually producing two images of each breast. The American College of Radiology (ACR) and the American Medical Association (AMA) recommend that women aged 40 years and over have a mammogram every year in order to ensure detection of breast cancer at an early stage.
Mammography, however, has several drawbacks that limit its detection ability and efficiency. Due to the two-dimensional nature of mammography, it is sometimes difficult to see a tumor that is obscured by dense tissue, such as the breast tissue of younger or pregnant women. Underlying and overlying breast tissue also can hide a tumor from view. In addition, the procedure for a mammogram requires the breast to be uncomfortably compressed between two plastic plates in order to produce a readable image. The use of x-rays, itself, is potentially harmful as patients receive a small amount of radiation during the mammogram, and ionizing radiation is known to increase cancer risk.
Digital tomosynthesis (DTS) is a pseudo 3-D x-ray imaging modality that has been extensively investigated for diagnostic imaging and image guidance in radiotherapy (Dobbins et al., Phys. Med. Biol. 48(19): R65-R106 (2003); Niklason et al., Radiology 205(2): 399-406 (1997); Suryanarayanan et al., Acad. Radiol. 7(12): 1085-1097 (2000); Wu et al., Med. Phys. 30(3): 365-380 (2003); and Wu et al., Int. J. Radiat. Oncol., Biol., Phys. 69(2): 598-606 (2007)). Like mammography, the procedure for DTS requires the breast to be compressed between two plastic plates while being imaged. Additionally, DTS requires more ionizing radiation to be used, since many projection images at different beam angles are necessary, and the resulting tomographic-like images are slices, which are parallel and close to the in-focus plane and moderate in quality.
It has been reported that magnetic resonance imaging (MRI) is more sensitive than mammography in identifying certain types of breast cancer (Kriege et al., N. Engl. J. Med. 351: 427-437 (2004); Lehman et al., J. Surg. Oncol. 92: 9-15 (2005a); Lehman et al., Cancer 103: 1898-1905 (2005b); and Leach et al., Lancet 365: 1769-1778 (2005)). Numerous clinical trials have demonstrated that MRI more accurately defines the true extent and type of breast disease than mammography alone or a mammography-ultrasound combination (Lehman et al. (2005a), supra; Lehman et al. (2005b), supra; Kuhl et al., J. Clin. Oncol. 23: 8469-8476 (2005)). However, due to the long imaging time and the associated high cost, MRI is only approved for screening high-risk patients. The false positive rate of MRI is also very high, leading to more unnecessary biopsies.
Ultrasound imaging is regularly used to supplement mammography in breast cancer screening because it provides high-resolution images with great soft tissue contrast compared with mammography. Given that it is non-invasive and does not involve the use of ionizing radiation, ultrasound is a widely used modality in medical imaging.
Existing ultrasound imaging methods, which can be used for clinical breast imaging, are designed to image a breast in the conventional reflection mode (B-mode). Such methods are commonly used to supplement mammograms because ultrasound can more easily differentiate between benign and cancerous masses. However, the dependence of conventional ultrasound on an operator has been a major obstacle for extensive use.
In the 1970s and 1980s researchers showed that it is also possible to construct breast images by measuring the transmission and refraction of ultrasound waves through the breast and calculating the speed of the ultrasound propagation at different locations in the breast (Greenleaf et al., in Acoustical Holography, edited by P. S. Green, Plenum, New York, volume 6, pages 71-90 (1975); Carson et al., Science 214: 1141-1143 (1981); Devaney, IEEE Trans. Biomed. Eng. BME-30: 377-386 (1983); and Devaney, Ultrason. Imaging 4: 336-350 (1982)). These multi-modal ultrasound techniques offer the promise of operator-independent, tomographic imaging of breast lesions. Proposed tomographic ultrasound imaging systems for single breast imaging have utilized these multiple ultrasound imaging modes.
Reflection imaging measures margins, or spans, of different masses in the breast. The images generated by reflection indicate changes on the surface of breast tissues. By analyzing these parameters in addition to the pulse-echo properties of ultrasound, accurate differentiation of benign and cancerous masses can be achieved. However, accurate differentiation with reflection imaging is operator-dependent and does not measure the transmission parameters, sound speed, and attenuation of the ultrasound wave in the breast.
Transmission imaging has shown itself to be an effective method of breast cancer screening that is operator-independent. In a study by Greenleaf et al. in 1977 (Ultrasonics Symposium: 989-995) in vitro samples were used to measure the sound speed and attenuation of different types of breast tissue. This study found that on a plot of sound speed vs. attenuation, malignant breast masses and benign masses have distinct sound speeds and attenuations. These findings indicate breast tissues of any density have sound speeds and attenuations that are very different from the sound speed and attenuation of cancerous masses. This makes transmission imaging a very valuable tool in differentiating between dense breast tissue and malignant breast lesions, which is hard to discern with mammography.
Whiting and Koch (U.S. Pat. No. 4,509,368) laid the ground work for tomographic ultrasound breast imaging. They disclosed a mechanism submersed in a water bath for scanning a single breast pendent in the water bath. It was envisioned that reflection, transmission, and sound speed could be used for tomographic reconstruction of the breast. Over the past 30 years, the work of others in the area has been described in numerous publications. For example, Shehada (U.S. Pat. No. 7,264,592 B2) discloses a tomographic ultrasound imaging apparatus for imaging a single breast that is submerged in a water tank.
Recently, Techniscan, Inc., constructed a functional multimodal ultrasound tomography system similar to that of Whiting and Koch. A clinical trial using the system for breast cancer screening has been conducted and shown great promise for imaging breast lesions. Techniscan uses two transducer arrays that revolve around the breast, one of which fires the transmission ultrasound waves, to collect data on both the transmitted and reflected waves. Using the collected data, they use inverse scattering techniques to reconstruct a 3-D image of the breast.
Another example of multimodal ultrasound breast imaging is the Computed Ultrasound Risk Evaluation system (C.U.R.E.) developed by Delphinus Technologies. C.U.R.E. uses a ring transducer, which eliminates the need to rotate the transducer array and also accounts for scatter in all directions along the image plane. In one imaging cycle of the C.U.R.E., all of the transducers take a turn firing an ultrasound signal, while the rest listen for the sound peak to arrive. Then, the ring translates downwards and repeats the process until the entire breast has been imaged. The use of reflection and transmission imaging while detecting 360 degrees of scatter makes this system capable of producing images with much higher resolution and greater specificity than mammography.
However, these systems have drawbacks that hinder their prevalence. For example, they use a water bath to submerge the entire breast, which can be messy and indecent to the patient. In addition, due to the size of the water tank, the patient must climb a ladder to get onto the imaging bed. Because these systems are large and somewhat inconvenient, they have not gained widespread clinical use.
In view of the above, there remains a need for a method, an apparatus, and a system of imaging a breast that does not suffer from the disadvantages attendant the use of the methods and the systems currently available in the art. An object of the present invention is to provide such a method, an apparatus, and a system. This and other objects and advantages, as well as inventive features, will become apparent from the detailed description provided herein.